Polymers, their preparation and uses

ABSTRACT

A polymer containing an optionally substituted repeat unit of formula (I) wherein each R is the same or different and represents H or an electron withdrawing group, and each R 1  is the same or different and represents a substituent.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of pending U.S. patent application Ser. No.13/889,308, filed May 7, 2013, which is a continuation of U.S. patentapplication Ser. No. 10/578,894, filed Jun. 8, 2007 (now abandoned),which is the U.S. national phase of PCT/GB2004/004754, filed Nov. 10,2004, which claims priority to Great Britain application no. 0326138.5filed Nov. 10, 2003, and Great Britain Application no. 0413205.6, filedJun. 14, 2004, all of which are expressly incorporated herein byreference and made a part hereof.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to polymers for electronic and opticalapplications and the synthesis thereof.

BACKGROUND OF THE INVENTION Related Technology

Organic semiconductors are attracting increasing attention across a widerange of applications due to their advantageous electronic propertiesand their processability. One class of opto-electrical devices is thatusing an organic material for light emission (an organic light-emissivedevice or “OLED”) or for light absorption for the purpose of powergeneration or light detection (a photovoltaic device). The basicstructure of these devices is a semiconducting organic layer, sandwichedbetween a cathode for injecting or accepting negative charge carriers(electrons) and an anode for injecting or accepting positive chargecarriers (holes) into or from the organic layer. For example, an OLED istypically fabricated on a glass or plastic substrate coated with atransparent first electrode such as indium-tin-oxide (“ITO”). A layer ofa thin film of at least one electroluminescent organic material coversthe first electrode. Finally, a cathode covers the layer ofelectroluminescent organic material. The cathode is typically a metal oralloy and may comprise a single layer, such as aluminium, or a pluralityof layers such as calcium and aluminium. Other layers can be added tothe device, for example to improve charge injection from the electrodesto the electroluminescent material. For example, a hole injection layersuch as poly(ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS)or polyaniline may be provided between the anode and theelectroluminescent material. In a practical device one of the electrodesis transparent, to allow the photons to escape or enter the device.

In the case of an OLED, holes are injected into the highest occupiedmolecular orbital (HOMO) of the electroluminescent material andelectrons are injected into its lowest unoccupied molecular orbital(LUMO). Holes and electrons then combine to generate excitons whichundergo radiative decay, the wavelength of emission being at leastpartially dependant on the HOMO-LUMO bandgap. Organic materials for useas light-emissive materials include polymers such aspoly(p-phenylenevinylene) (as disclosed in WO 90/13148), polyfluorenesand polyphenylenes; the class of materials known as small moleculematerials such as tris-(8-hydroxyquinoline)aluminium (“Alq₃”) asdisclosed in U.S. Pat. No. 4,539,507; and the class of materials knownas dendrimers as disclosed in WO 99/21935. These materialselectroluminesce by radiative decay of singlet excitons (i.e.fluorescence) however spin statistics dictate that up to 75% of excitonsare triplet excitons which undergo non-radiative decay, i.e. quantumefficiency may be as low as 25% for fluorescent OLEDs and so thesematerials or similar materials capable of transporting charge may beused as hosts for dopants comprising heavy metal complexes capable ofharvesting triplet excitons for radiative decay (phosphorescence) asdisclosed in, for example, Pure Appl. Chem., 1999, 71, 2095, MaterialsScience & Engineering, R: Reports (2002), R39(5-6), 143-222 andPolymeric Materials Science and Engineering (2000), 83, 202-203.

Polyfluorenes having a repeat unit of formula (A) are disclosed in forexample, Adv. Mater. 2000 12(23) 1737-1750:

wherein R″ represents a solubilizing group such as n-octyl.

These polymers have attracted considerable interest aselectroluminescent materials because they are solution processable andhave good film forming properties. Furthermore, these polymers may bemade by Yamamoto or Suzuki polymerization, for which the appropriatemonomers are accessed simply by halogenation of fluorene to form a2,7-dihalofluorene. These polymerization techniques enablepolymerization of fluorene monomers with a wide range of aromaticco-monomers and afford a high degree of control over regioregularity ofthe polymer. Thus, the physical and electronic properties ofpolyfluorenes may be tailored by appropriate selection of monomers.

Linkage of the fluorene repeat units through the 2- and 7-positions isimportant for maximization of conjugation through the repeat unit.

A focus in the field of PLEDs has been the development of full colordisplays for which red, green and blue electroluminescent polymers arerequired—see for example Synthetic Metals 111-112 (2000), 125-128. Tothis end, a large body of work has been reported in the development ofelectroluminescent polymers for each of these three colors with red,green and blue emission as defined by PAL standard 1931 CIEco-ordinates.

A difficulty encountered with blue electroluminescent polymers to dateis that their lifetime (i.e. the time taken for brightness to halve froma given starting brightness at fixed current) tends to be shorter thanthat of corresponding red or green materials. One of the factors thathas been proposed as contributing to the more rapid degradation of bluematerials is that their LUMO levels, and consequently the energy levelof the charged state following injection of an electron into the LUMO,tend to be less deep (i.e. relatively low electron affinity) than thoseof corresponding red or green materials. It is therefore possible thatmaterials comprising these lower electron affinities are lesselectrochemically stable and so more prone to degradation.

For simplicity, a full color display will preferably have a commoncathode material for all three electroluminescent materials. Thus, theproblem of a large energy gap between the LUMO and the workfunction ofthe cathode for a typical blue electroluminescent material is likely tobe exacerbated where a common cathode suitable for red and greenmaterials is employed.

A blue electroluminescent material having a higher electron affinitythan polyfluorenes or a material capable of injecting electrons intoblue electroluminescent polymers is therefore desirable, howeverincreasing the electron affinity of a wide bandgap material will tend toresult in a smaller bandgap thus making the material less suitable as ablue emitter or as an electron transporting material for a blue emitter.

A further drawback of polyfluorenes is that blue electroluminescentpolyfluorenes have a tendency to shift over time towards longerwavelengths, i.e. towards a redder colour of emission. This effect isbelieved to be due to oxidative degradation and aggregation of thepolymer.

EP 1318163 discloses a monomer of formula (B), and electroluminescentpolymers derived therefrom:

Likewise, JP 2003-206289 discloses a monomer of formula (C) and polymersderived therefrom:

These disclosures teach formation of the above dibenzosilole monomerseither

via (a) lithiation of the 2- and 7-positions of the correspondingnon-halogenated compound followed by halogen exchange, or (b) by thefollowing process:

In case (a), the alkoxy groups serve to direct lithiation at theadjacent 2- and 7-positions. Likewise, in case (b) the alkoxy groupsserve to direct bromination in the same way. Although these alkoxygroups are significant in monomer synthesis, they are likely to causerepeat units derived from such monomers to suffer from stericinterference with adjacent repeat units resulting in a twist in thepolymer backbone and loss of conductivity. Furthermore, the electrondonating nature of these alkoxy groups decreases the electron affinityof polymers derived from these monomers.

A further drawback of polymers derived from monomers (B) and (C) is thatthe phenyl and methyl groups of these monomer do not afford solubilityin common organic solvents such as xylene.

It is therefore an object of the invention to provide a wide bandgappolymer having higher electron affinity than a polyfluorene, i.e. amaterial capable of blue emission and capable of serving as an electrontransporting material for other blue and smaller bandgap emissivematerials. It is a further object of the invention to provide such apolymer that does not suffer from undesirable steric effects; that doesnot suffer from a color shift over time; and that is readily soluble incommon organic solvents. It is a yet further object of the invention toprovide a host material for luminescent dopants, in particularphosphorescent dopants.

SUMMARY OF THE INVENTION

The present inventors have found a novel class of dibenzosiloles thatsolve the aforementioned drawbacks of polyfluorenes.

Accordingly, in a first aspect the invention provides a polymercomprising an optionally substituted first repeat unit of formula (I):

wherein each R is the same or different and represents H or an electronwithdrawing group; and each R¹ is the same or different and represents asubstituent.

Preferred electron withdrawing groups are selected from: groupscomprising fluorine, cyano, nitro, carboxyl, amides, ketones,phosphinoyl, phosphonates, sulfones and esters. Preferred groupscomprising fluorine include fluorine atoms, fluoroalkyl, fluoroaryl andfluoroheteroaryl.

Other electron withdrawing groups R will be apparent to the skilledperson. In particular, those substituents having a positive Hammettsigma constant are suitable.

The present inventors have found that polymers according to the firstaspect of the invention are high electron affinity, wide bandgapmaterials. In contrast to the prior art, polymers according to theinvention do not possess electron-donating groups in the 3- and6-positions that lessen the electron affinity of the dibenzosiloleunits.

In order to avoid steric interactions between adjacent repeat units, itis preferred that at least one R group is hydrogen. More preferably,both R groups are hydrogen.

Preferably, at least one R¹ is a solubilizing group.

Preferably, each R¹ is the same or different and is selected from thegroup consisting of optionally substituted C₁₋₂₀ alkyl, C₁₋₂₀ alkoxy,aryl and heteroaryl. More preferably, each R¹ is independently a C₄₋₁₀alkyl, most preferably n-hexyl or n-octyl. The optional substituents,where present, are preferably electron withdrawing groups, in particularfluorine. One or more such substituents may be provided on each R¹group.

Preferably, the polymer comprises an optionally substituted aryl orheteroaryl second repeat unit.

In a second aspect, the invention provides a monomer of formula (II):

wherein R and R¹ are as described in the first aspect of the inventionand each X is the same or different and represents a polymerisablegroup.

Preferably, each X is the same or different and is selected from thegroup consisting of boronic acid groups, boronic ester groups, boranegroups and halide functional groups.

In a third aspect, the invention provides a method of forming a polymercomprising the step of polymerizing the monomer of formula (II).

Preferably, each X is independently selected from the group consistingof boronic acid groups, boronic ester groups and borane groups andhalide functional groups and the polymerization is performed in thepresence of a transition metal catalyst.

In one preferred embodiment of the third aspect, each X is the same ordifferent and is a halide functional group, and the polymerization isperformed in the presence of a nickel complex catalyst.

In another preferred embodiment of the third aspect, the methodcomprises polymerizing:

a monomer of formula (II) wherein each X is the same or different and isa boron derivative functional group selected from a boronic acid, aboronic ester and a borane, and an aromatic monomer having at least tworeactive halide functional groups; or

a monomer of formula (II) wherein each X is the same or different and isa reactive halide functional group, and an aromatic monomer having atleast two boron derivative functional group selected from a boronicacid, a boronic ester and a borane; or

a monomer of formula (II) wherein one X is a reactive halide functionalgroup and the other X is a boron derivative functional group selectedfrom a boronic acid, a boronic ester and a borane,

wherein the reaction mixture comprises a catalytic amount of a palladiumcatalyst suitable for catalyzing the polymerization of the aromaticmonomers, and a base in an amount sufficient to convert the boronderivative functional groups into boronate anionic groups.

In a fourth aspect, the invention provides an optical device comprisinga polymer according to the first aspect of the invention.

Preferably, the optical device comprises an anode, a cathode and a layerof the polymer according to the first aspect of the invention locatedbetween the anode and the cathode.

Preferably, the optical device is an electroluminescent device.

In a fifth aspect, the invention provides a switching device comprisinga polymer according to the first aspect of the invention.

Preferably, the switching device is a thin film transistor.

The present inventors have found that monomers of formula (II) may beformed from a class of key intermediates that do not require thepresence of ortho-directing groups.

Accordingly, in a sixth aspect the invention provides an optionallysubstituted compound of formula (IV):

wherein each X¹ and each X² is the same or different and represents aleaving group capable of participating in a transmetallation reactionand X² has an electronegativity less than that of X¹.

Preferably, both X¹ groups are the same and both X² groups are the same.Where the two groups X¹ and/or the two groups X² are different, it willbe appreciated that the electronegativity of the least electronegativeX¹ group shall be greater than the electronegativity of the mostelectronegative X² group.

Preferably, each X¹ and X² is independently a halogen. More preferably,X¹ and X² are selected from bromine, chlorine and iodine. Mostpreferably, X¹ is bromine and X² is iodine.

The compound of formula (IV) serves as an intermediate to a variety ofmonomers including but not limited to dibenzosiloles.

Accordingly, in a seventh aspect the invention provides a method offorming a monomer of formula (VI) from a compound of formula (V)according to the following scheme:

wherein the method comprises reacting the compound of formula (V) with atransmetallating agent followed by reaction with a compound of formulaLG-Y-LG, wherein X¹ and R are as defined in the sixth aspect of theinvention; each X³ is the same or different and represents a leavinggroup capable of participating in a transmetallation having anelectronegativity less than or the same as that of X¹; Y represents adivalent residue comprising a backbone of 1-3 atoms; and each LG is thesame or different and represents a leaving group.

By “transmetallating agent” is meant a compound capable of reacting withthe C—X² bond of the compound of formula (IV) to transform it into acarbon-metal bond.

Preferably, Y comprises a single atom in its backbone selected from thegroup consisting of —CR³ ₂—, —SiR³ ₂—, —NR³—, —PR³—, —GeR³ ₂—, —SnR³ ₂—,O and S, wherein R³ is selected from the group consisting of optionallysubstituted alkyl, alkoxy, aryl and heteroaryl. More preferably, Y isselected from the group consisting of —CR³ ₂—, —SiR³ ₂—, —NR³—, —PR³—,—GeR³ ₂—, —SnR³ ₂—, O and S. Preferably, each R³ is the same ordifferent and is a C₁₋₂₀ alkyl.

Preferably, each LG is the same or different and is a halogen, morepreferably chlorine, bromine or iodine.

Preferably, the transmetallating agent is a compound of formula R⁴-Mwherein R⁴ is alkyl or aryl and M is a metal. Preferably, M is lithium.Preferably, R⁴ is C₁₋₄ alkyl or phenyl.

As outlined above, linkage of dibenzosilole repeat units according tothe invention through their 2- and 7-positions maximises conjugation ofpolymer chains comprising these repeat units. However, the presentinventors have found that non-2,7-linked dibenzosiloles possess a widerbandgap than corresponding 2,7-linked dibenzosiloles and at the sametime retain a desirable high electron affinity. Moreover, non-2,7-linkeddibenzosiloles have been found to possess a higher triplet energy levelthan corresponding 2,7-linked dibenzosiloles and as such may serve asthe host material for a wider range of fluorescent or phosphorescentdopants.

Accordingly, in an eighth aspect the invention provides a polymercomprising an optionally substituted first repeat unit of formula (VII):

wherein each R² is the same or different and represents a substituent;the R² groups may be linked to form a ring; and bond (a) is not linkedto the 2-position of the repeat unit of formula (VII).

Bond (b) may or may not be bound to the 7-position of the repeat unit offormula (VII), however in a preferred embodiment bond (b) is not boundto the 7-position of the repeat unit of formula (VII).

Preferably, bond (a) is bound to the 3-position of the repeat unit offormula (VII).

Preferably, bond (b) is bound to the 6-position of the repeat unit offormula (VII).

Preferably, at least one R² is a solubilizing group.

Preferably, each R² is the same or different and is selected from thegroup consisting of optionally substituted C₁₋₂₀ alkyl, C₁₋₂₀ alkoxy,aryl and heteroaryl. More preferably, each R² is independently a C₄₋₁₀alkyl, most preferably n-hexyl or n-octyl.

Preferably, the polymer comprises an optionally substituted aryl orheteroaryl second repeat unit.

In a ninth aspect the invention provides an optionally substitutedmonomer of formula (VIII):

wherein each R² is as defined in the eighth aspect of the invention;each X is as defined in the second aspect of the invention and at leastone X is not linked to the 2-position of the repeat unit of formula(VIII).

The monomer of formula (VIII) may be polymerised in accordance with themethod described in the third aspect of the invention.

The present inventors have found that polymers comprising dibenzosilolerepeat units function very effectively as host materials for luminescentdopants.

Accordingly, in a tenth aspect the invention provides anelectroluminescent device comprising an anode, a cathode and anelectroluminescent layer located between the anode and cathode whereinthe electroluminescent layer comprises a polymeric host materialcomprising an optionally substituted first repeat unit of formula (IX)and a luminescent dopant

wherein R¹ is as described in the first aspect of the invention.

Preferably, the repeat unit of formula (IX) is not linked through its2-position. More preferably, the repeat unit of formula (IX) is notlinked through its 2- or 7-positions. Most preferably, the repeat unitof formula (IX) is linked through its 3- and 6-positions.

Preferably, the polymer comprises a second repeat unit. Preferably, thesecond repeat unit comprises a hole transporting material. Morepreferably, the second repeat unit is a carbazole, more preferably a3,6-linked carbazole.

The luminescent dopant may be phosphorescent or fluorescent. Preferably,the luminescent dopant is phosphorescent.

Preferably, the repeat unit of formula (IX) is unsubstituted.

Reaction of dibenzosiloles with organolithium reagents is disclosed inJ. Organomet. Chem., 1983, 250, 109-119. In particular, transalkylationof 1-methyl-1-(trimethylsilyl)- and1-methyl-1-(trimethylsilyl)-dibenzosilole with methyllithium,butyllithium and phenyllithium is disclosed. Angew. Chem. Int. Ed.,1996, 35, 1127-1128 indicates that such reactions proceed via apentaco-ordinate intermediate.

The dibenzosiloles disclosed in this prior art do not carry any reactivegroups other than at the silicon atom of the dibenzosilole wheretransalkylation takes place. The present inventors have surprisinglyfound that the substituents carried at the silicon atom ofdibenzosiloles may selectively be changed, e.g. by transalkylation, evenwhen reactive substituents are present elsewhere on the dibenzosilole.

Accordingly, in an eleventh aspect the invention provides a method offorming an optionally substituted compound of formula (X) according tothe following process:

wherein each R⁸ is independently selected from the group consisting ofC₁₋₂₀ alkyl and aryl; each R⁹ is different from R⁸ and is independentlyselected from the group consisting of C₁₋₂₀ alkyl, aryl and heteroaryl;M¹ is a metal; and Z is a reactive group capable of undergoing reactionwith M¹-R⁹.

Preferably, M¹ is lithium.

Preferably, R⁸ is methyl.

Preferably, Z is trialkylsilyl, more preferably trimethylsilyl.

In case of reaction with M¹-R⁹ the two groups R¹⁰ are not linked to forma ring. Preferred groups R⁹ in this case are C₄₋₂₀ alkyl.

In case of reaction with M¹-R⁹—R⁹-M¹ the two groups R¹⁰ are linked toform a ring. In this case, preferred groups R⁹—R⁹ are C₄₋₂₀ alkylene oroptionally substituted biaryl or bi-heteroaryl, in particular biphenyl.

A particularly preferred compound of formula M¹-R⁹—R⁹-M¹ has thestructure shown below, which may optionally be substituted:

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows a prior art electroluminescent device

FIG. 2(a) shows a plot of photoluminescent wavelength over time for aprior art polyfluorene

FIG. 2(b) shows a plot of photoluminescent wavelength over time for aco-polymer according to the invention

FIG. 2(c) shows a plot of photoluminescent wavelength over time for ahomopolymer according to the invention

FIG. 3 shows a plot of electroluminescent wavelength for polymersaccording to the invention as compared to a prior art polyfluorene

DETAILED DESCRIPTION OF THE INVENTION

A polymer according to the present invention may comprise a homopolymeror copolymers (including terpolymers or higher order polymers).

Copolymers according to the present invention include regularalternating, random and block polymers where the percentage of eachmonomer used to prepare the polymer may vary.

Preferred co-repeat units include triarylamines, arylenes andheteroarylenes.

Examples of arylene repeat units are fluorene, particularly 2,7-linked9,9 dialkyl fluorene or 2,7-linked 9,9 diaryl fluorene; spirofluorenesuch as 2,7-linked 9,9-spirofluorene; indenofluorene such as a2,7-linked indenofluorene; or phenyl such as alkyl or alkoxy substituted1,4-phenylene. Each of these groups may optionally be substituted.

Particularly preferred triarylamine repeat units derived fromtriarylamine monomers include units of formulae 1-6:

A and B may be the same or different and are substituent groups. It ispreferred that one or both of A and B is independently selected from thegroup consisting of alkyl, aryl, perfluoroalkyl, thioalkyl, cyano,alkoxy, heteroaryl, alkylaryl and arylalkyl groups. One or more of A andB also may be hydrogen. It is preferred that one or more of A and B isindependently an unsubstituted, isobutyl group, an n-alkyl, an n-alkoxyor a trifluoromethyl group because they are suitable for helping toselect the HOMO level and/or for improving solubility of the polymer.

Particularly preferred heteroaryl repeat units include units of formulae7-21:

wherein R₆ and R₇ are the same or different and are each independentlyhydrogen or a substituent group, preferably alkyl, aryl, perfluoroalkyl,thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl or arylalkyl. For easeof manufacture, R₆ and R₇ are preferably the same. More preferably, theyare the same and are each a phenyl group.

Further suitable Ar groups are known in this art, for example asdisclosed in WO 00/55927 and WO 00/46321, the contents of which areincorporated herein by reference.

For ease of processing, it is preferred that the polymer is soluble.Substituents such as C₁₋₁₀ alkyl or C₁₋₁₀alkoxy may usefully be selectedto confer on the polymer solubility in a particular solvent system.Typical solvents include mono- or poly-alkylated benzenes such astoluene and xylene or tetrahydrofuran. Techniques for solutiondeposition of the polymer according to the invention include inkjetprinting as disclosed in EP 0880303, spin-coating, dip-coating, doctorblade coating and screen printing.

The polymers according to the invention may carry cross-linkable groupssuch as oxetanes, azides, acrylates, vinyl and ethynyl groups in orderthat the polymer may be deposited in a soluble form followed bycross-linking to render the polymer insoluble. Cross-linking may beachieved through thermal treatment or exposure of the polymer toradiation, in particular UV radiation. Cross-linking may be employed toallow deposition of multiple layers from solution as disclosed in WO96/20253. Alternatively, photo-initiated cross-linking may be used byexposure of a polymer layer through a mask to form a pattern ofinsoluble material from which unexposed, soluble polymer may be removedby solvent treatment as disclosed in Nature 421, 829-833, 2003.

Two polymerisation techniques that are particularly amenable topreparation of conjugated polymers from aromatic monomers such asdibenzosilole monomers according to the invention are Suzukipolymerisation as disclosed in, for example, WO 00/53656 and Yamamotopolymerisation as disclosed in, for example, “Macromolecules”, 31,1099-1103 (1998). Suzuki polymerisation entails the coupling of halideand boron derivative functional groups; Yamamoto polymerisation entailsthe coupling of halide functional groups. Accordingly, it is preferredthat each monomer is provided with two reactive functional groupswherein each functional group is independently selected from the groupconsisting of (a) boron derivative functional groups selected fromboronic acid groups, boronic ester groups and borane groups and (b)halide functional groups.

The transmetallating agents used to prepare monomers of the inventioninclude alkyl- and aryl-lithium compounds such as methyllithium,n-butyllithium, t-butyllithium, phenyllithium and lithium di-isopropylamine.

Examples of monomers of formula (VI) preparable according to the methodof the invention include the following:

With reference to FIG. 1, the standard architecture of an optical deviceaccording to the invention, in particular an electroluminescent device,comprises a transparent glass or plastic substrate 1, an anode of indiumtin oxide 2 and a cathode 4. The polymer according to the invention islocated in layer 3 between anode 2 and cathode 4. Layer 3 may comprisethe polymer according to the invention alone or a plurality of polymers.Where a plurality of polymers are deposited, they may comprise a blendof at least two of a hole transporting polymer, an electron transportingpolymer and, where the device is a PLED, an emissive polymer asdisclosed in WO 99/48160. Alternatively, layer 3 may be formed from asingle polymer that comprises regions selected from two or more of holetransporting regions, electron transporting regions and emissive regionsas disclosed in, for example, WO 00/55927 and U.S. Pat. No. 6,353,083.Each of the functions of hole transport, electron transport and emissionmay be provided by separate polymers or separate regions of a singlepolymer. Alternatively, more than one function may be performed by asingle region or polymer. In particular, a single polymer or region maybe capable of both charge transport and emission. Each region maycomprise a single repeat unit, e.g. a triarylamine repeat unit may be ahole transporting region. Alternatively, each region may be a chain ofrepeat units, such as a chain of polyfluorene or dibenzosilole units asan electron transporting region. The different regions within such apolymer may be provided along the polymer backbone, as per U.S. Pat. No.6,353,083, or as groups pendant from the polymer backbone as per WO01/62869.

In addition to layer 3, a separate hole transporting layer and/or anelectron transporting layer may be provided.

The polymers according to the invention may be used as the host for afluorescent dopant as disclosed in, for example, J. Appl. Phys. 65,3610, 1989 or as the host for a phosphorescent dopant as disclosed in,for example, Nature (London), 1998, 395, 151.

Preferred metal complexes comprise optionally substituted complexes offormula (XI):

M¹L¹ _(q)L² _(r)L³ _(s)   (XI)

wherein M¹ is a metal; each of L¹, L² and L³ is a coordinating group; qis an integer; r and s are each independently 0 or an integer; and thesum of (a. q)+(b. r)+(c.s) is equal to the number of coordination sitesavailable on M, wherein a is the number of coordination sites on L¹, bis the number of coordination sites on L² and c is the number ofcoordination sites on L³.

Heavy elements M induce strong spin-orbit coupling to allow rapidintersystem crossing and emission from triplet states (phosphorescence).Suitable heavy metals M include:

lanthanide metals such as cerium, samarium, europium, terbium,dysprosium, thulium, erbium and neodymium; and

d-block metals, in particular those in rows 2 and 3 i.e. elements 39 to48 and 72 to 80, in particular ruthenium, rhodium, palladium, rhenium,osmium, iridium, platinum and gold.

Suitable coordinating groups for the f-block metals include oxygen ornitrogen donor systems such as carboxylic acids, 1,3-diketonates,hydroxy carboxylic acids, Schiff bases including acyl phenols andiminoacyl groups. As is known, luminescent lanthanide metal complexesrequire sensitizing group(s) which have the triplet excited energy levelhigher than the first excited state of the metal ion. Emission is froman f-f transition of the metal and so the emission colour is determinedby the choice of the metal. The sharp emission is generally narrow,resulting in a pure color emission useful for display applications.

The d-block metals form organometallic complexes with carbon or nitrogendonors such as porphyrin or bidentate ligands of formula (XII):

wherein Ar¹ and Ar² may be the same or different and are independentlyselected from optionally substituted aryl or heteroaryl; Z¹ and Z² maybe the same or different and are independently selected from carbon ornitrogen; and Ar¹ and Ar² may be fused together. Ligands wherein Z¹ iscarbon and Z² is nitrogen are particularly preferred.

Examples of bidentate ligands are illustrated below:

Each of Ar¹ and Ar² may carry one or more substituents. Particularlypreferred substituents include fluorine or trifluoromethyl which may beused to blue-shift the emission of the complex as disclosed in WO02/45466, WO 02/44189, US 2002-117662 and US 2002-182441; alkyl oralkoxy groups as disclosed in JP 2002-324679; carbazole which may beused to assist hole transport to the complex when used as an emissivematerial as disclosed in WO 02/81448; bromine, chlorine or iodine whichcan serve to functionalise the ligand for attachment of further groupsas disclosed in WO 02/68435 and EP 1245659; and dendrons which may beused to obtain or enhance solution processability of the metal complexas disclosed in WO 02/66552.

Other ligands suitable for use with d-block elements includediketonates, in particular acetylacetonate (acac); triarylphosphines andpyridine, each of which may be substituted.

Main group metal complexes show ligand based, or charge transferemission. For these complexes, the emission color is determined by thechoice of ligand as well as the metal. A wide range of fluorescent lowmolecular weight metal complexes are known and have been demonstrated inorganic light emitting devices [see, e. g., Macromol. Sym. 125 (1997)1-48, U.S. Pat. No. 5,150,006, U.S. Pat. No. 6,083,634 and U.S. Pat. No.5,432,014], in particular tris-(8-hydroxyquinoline)aluminium. Suitableligands for di or trivalent metals include: oxinoids, e. g. withoxygen-nitrogen or oxygen-oxygen donating atoms, generally a ringnitrogen atom with a substituent oxygen atom, or a substituent nitrogenatom or oxygen atom with a substituent oxygen atom such as8-hydroxyquinolate and hydroxyquinoxalinol-10-hydroxybenzo (h)quinolinato (II), benzazoles (III), Schiff bases, azoindoles, chromonederivatives, 3-hydroxyflavone, and carboxylic acids such as salicylatoamino carboxylates and ester carboxylates. Optional substituents includehalogen, alkyl, alkoxy, haloalkyl, cyano, amino, amido, sulfonyl,carbonyl, aryl or heteroaryl on the (hetero) aromatic rings which maymodify the emission color.

The metal complex may be incorporated into the host polymer of theinvention, either as a substituent on the main chain of the polymer orincorporated into the main chain of the polymer, as disclosed in, forexample, EP 1245659, WO 02/31896, WO 03/18653 and WO 03/22908. In thiscase, the polymer may provide the functions of emission and at least oneof hole transport and electron transport.

The host polymer of the invention may be a homopolymer or a copolymer.In the case of copolymers, suitable co-repeat units include carbazoles,such as 2,7-linked carbazole repeat units. Alternatively, 3,6-linkedcarbazole repeat units as disclosed in J. Am. Chem. Soc. 2004, 126,6035-6042 may also be used.

It is reported in J. Am. Chem. Soc. 2004, 126, 6035-6042 that shorterpoly paraphenylene chains within the host backbone increases the tripletenergy level of the host. It will therefore be appreciated that a highertriplet energy level for copolymers comprising a repeat unit accordingto the invention is achieved using a 3,6-linked carbazole repeat unitand a 3,6-linked dibenzosilole repeat unit according to the eighthaspect of the invention, as compared to a copolymer with units linkedthrough 2,7-positions. Similarly, see “Carbazole Compounds as HostMaterials for Triplet Emitters in Organic Light Emitting Diodes: PolymerHosts for High Efficiency Light Emitting Diodes” Addy van Dijken,Jolanda J. A. M. Bastiaansen, Nicole M. M. Kiggen, Bea M. W. Langeveld,Carsten Rothe, Andy Monkman, Ingrid Bach, Philipp Stössel, and KlemensBrunner, J. Am. Chem. Soc. ASAP Articles, Web Release Date: 28 May 2004.Accordingly, a preferred host material has formula (XIII):

wherein R¹ is as defined in the first aspect of the invention. Polymersof this type may comprise blocks of the illustrated dibenzosilole andcarbazole units, however in a particularly preferred embodiment thepolymer (XIII) is a 1:1 copolymer of alternating dibenzosilole andcarbazole units.

Other suitable co-repeat units for host co-polymers according to theinvention include triarylamine repeat units of formulae 1-6 describedabove. Accordingly, another preferred host polymer has formula (XIV):

wherein R¹ is as defined in the first aspect of the invention and A isas defined above. Polymers of this type may comprise blocks of theillustrated dibenzosilole and triarylamine units, however in aparticularly preferred embodiment the polymer (XIV) is a 1:1 copolymerof alternating dibenzosilole and carbazole units.

Although not essential, a layer of organic hole injection material (notshown) between the anode 2 and the polymer layer 3 is desirable becauseit assists hole injection from the anode into the layer or layers ofsemiconducting polymer. Examples of organic hole injection materialsinclude poly(ethylene dioxythiophene) (PEDT/PSS) as disclosed in EP0901176 and EP 0947123, or polyaniline as disclosed in U.S. Pat. No.5,723,873 and U.S. Pat. No. 5,798,170.

Cathode 4 is selected from materials that have a workfunction allowinginjection of electrons into the electroluminescent layer. Other factorsinfluence the selection of the cathode such as the possibility of theadverse interactions between the cathode and the electroluminescentmaterial. The cathode may consist of a single material such as a layerof aluminium. Alternatively, it may comprise a plurality of metals, forexample a bilayer of calcium and aluminium as disclosed in WO 98/10621,elemental barium disclosed in WO 98/57381, Appl. Phys. Lett. 2002,81(4), 634 and WO 02/84759 or a thin layer of dielectric material toassist electron injection, for example lithium fluoride disclosed in WO00/48258 or barium fluoride, disclosed in Appl. Phys. Lett. 2001, 79(5),2001.

A typical electroluminescent device comprises an anode having aworkfunction of 4.8 eV. Accordingly, the HOMO level of the holetransporting region is preferably around 4.8-5.5 eV. Similarly, thecathode of a typical device will have a workfunction of around 3 eV.Accordingly, the LUMO level of the electron transporting region ispreferably around 3-3.5 eV.

The polymers according to the invention may also be used in currentswitching devices for an integrated circuit as disclosed in, forexample, WO 99/54936. In particular, the polymer may be a component of afield effect transistor comprising an insulator with a gate electrodelocated on one side of the insulator; a polymer according to theinvention located on the other side of the insulator; and a drainelectrode and a source electrode located on the polymer.

Electroluminescent devices may be monochrome devices or full colourdevices (i.e. formed from red, green and blue electroluminescentmaterials).

EXAMPLES A) 2,7-Linked Dibenzosilole Monomers and Repeat Units MonomerExample

Monomers 1 and 2 according to the second aspect of the invention wassynthesised according to the following reaction scheme.

a) Cu, DMF, 125° C., 3 h, 75%; b) Sn, HCl, EtOH, 110° C., 2 h, 72%; c)(i) NaNO₂, HCl, 0° C., 1 h (ii) KI, −10 to 50° C., 2 h, 15%; d) (i)t-BuLi, THF, −90 to −78° C., 2 h (ii) Si(n-hexyl)₂Cl₂, 24 h, 52%; e) (i)t-BuLi, diethyl ether, −78° C., 1 h (ii)2-isopropoxy-4,4′,5,5′-tetramethyl-1,3,2-dioxaboralane, roomtemperature, 24 h, 74%.

4,4′-Dibromo-2,2′-dinitro-biphenyl

Reference: R. G. R. Bacon and S. G. Pande, J. Chem. Soc., 1970, 1967.

To a solution of 2,5-dibromonitrobenzene (50.0 g, 179 mmol) in DMF (200cm³) was added copper powder (27.0 g, 424 mmol) and the reaction mixtureheated to 125° C. After 3 h, the mixture was allowed to cool to roomtemperature and then treated with toluene (200 cm³). The insolubleinorganic salts were removed by filtration through celite and thefiltrate was evaporated to dryness. The crude material was vigorouslywashed with methanol (500 cm³) and redissolved in toluene (200 cm³). Theremaining inorganic salts were again removed by filtration throughcelite, and the filtrate was evaporated to yield the title compound(27.1 g, 75%) as yellow crystals (Found: C, 35.8; H, 1.5; N, 6.7.C₁₂H₆Br₂N₂O₄ requires C, 35.9; H, 1.5; N, 7.0%); ν_(max)/cm⁻¹ (Neatsolid) 730, 829, 1004, 1103, 1344, 1526; δ_(H)(500 MHz, CDCl₃) 7.18 (2H,d, J 8.2, ArH), 7.85 (2H, dd, J 8.2, 2.0, ArH), 8.39 (2H, d, J 2.0,ArH); δ_(C)(100 MHz, CDCl₃) 122.9, 128.1, 131.9, 132.0, 136.6, 147.4.

4,4′-Dibromo-biphenyl-2,2′-diamine

Reference: Patrick, D. A.; Boykin, D. W.; Wilson, W. D.; Tanious, F. A.;Spychala, J.; Bender, B. C.; Hall, J. E.; Dykstra, C. C.; Ohemeng, K.A.; Tidwell, R. R., Eur. J. Med. Chem., 1997, 32(10), 781.

To a solution of 4,4′-Dibromo-2,2′-dinitro-biphenyl (15.0 g, 37.3 mmol)in ethanol (abs., 186 cm³) was added 32% w/w aqueous HCl (124 cm³). Tinpowder (17.6 g, 147 mmol) was added portion-wise over 10 minutes and thereaction mixture was heated to reflux at 100° C. for 2 hours. Aftercooling, the mixture was poured into ice water (ca. 400 cm³) and thenbasified with 20% w/w aqueous NaOH solution (150 cm³). The product wasextracted with diethyl ether and the organic layer washed with brine,dried over anhydrous MgSO₄ and evaporated to dryness. Purification byrecrystallization from ethanol afforded the title compound (9.2 g, 72%)as light brown crystals (Found: C, 42.1; H, 3.0; N, 8.0. C₁₂H₁₀Br₂N₂requires C, 42.2; H, 3.0; N, 8.2%); ν_(max)/cm⁻¹ (Neat solid) 792, 994,1406, 1477, 1608, 3210, 3357, 3443; δ_(H) (400 MHz, CDCl₃) 6.92 (6H, s,ArH), 3.78 (4H, brs, NH₂); δ_(C)(100 MHz, CDCl₃) 118.1, 121.7, 122.0,122.7, 132.2, 145.4; m/z (ES) 340.9283 ([M+H)⁺. C₁₂H₁₁Br₂N₂ requires340.9284), 343.1 (100%), 263.1 (80), 185.1 (25).

4,4′-Dibromo-2,2′-diiodo-biphenyl

4,4′-Dibromo-biphenyl-2,2′-diamine (5.0 g, 14.6 mmol) was suspended in16% w/w aqueous. HCl (16 cm³) at 0° C. Sodium nitrite (2.2 g, 31.9 mmol)was added dropwise whilst maintaining the temperature at 0° C. After afurther 60 minutes of stirring at 0° C., KI solution (5.0 g, 30.1 mmolin 5 cm³ H₂O) was added dropwise to the reaction mixture at −10° C. Thereaction mixture was allowed to warm to room temperature, and then to50° C. for 2 h. The crude reaction mixture was allowed to cool to roomtemperature and then basified with 10% w/w aqueous NaOH (90 cm³). Theproduct was extracted into diethyl ether and the organic layer washedwith brine, dried with anhydrous MgSO₄ and evaporated. Purification bycolumn chromatography (hexane) yielded the title compound (1.45 g, 15%)as an off-white solid (Found: C, 25.8; H, 1.0. C₁₂H₆Br₂I₂ requires C,25.6; H, 1.1%); mp 89° C.; ν_(max)/cm⁻¹ (Neat solid) 710, 817, 993,1086, 1448, 1565; δ_(H)(400 MHz, CDCl₃) 7.03 (2H, d, J 8.2, ArH), 7.55(2H, dd, J 8.2 1.9, ArH), 8.08 (2H, d, J 1.9, ArH); δ_(C)(100 MHz,CDCl₃) 99.8, 122.5, 130.7, 131.4, 141.0, 146.8.

2,7-Dibromo-9,9′-dihexyl-9H-9-dibenzosiloledibenzosilole

t-Butyllithium (6.26 cm³, 10.6 mmol, 1.7 M in Pentane) was added over 2h to a solution of 4,4′-dibromo-2,2′-diiodo-biphenyl (1.5 g, 2.66 mmol)in dry THF (30 cm³) at −90° C. under nitrogen atmosphere. The mixturewas stirred for a further 1 h at −90° C. Dichlorodihexylsilane wassubsequently added and the mixture was stirred at room temperatureovernight. The reaction was quenched with distilled water, and the THFwas removed by vacuum. The product was then extracted into diethyl etherand the organic layer washed with brine, dried with anhydrous MgSO₄ andevaporated. Purification by column chromatography (hexane) yielded thetitle compound (0.7 g, 52%) as a colorless oil; ν_(max)/cm⁻¹ (Neatliquid) 720, 813, 1001, 1072, 1384, 2855, 2923, 2956; δ_(C) (500 MHz,CDCl₃) 0.84-0.97 (10H, m, CH₂+CH₃), 1.22-1.36 (16H, m, CH₂), 7.55 (2H,dd, J 8.3 2.0, ArH), 7.64 (2H, d, J 8.3, ArH), 7.70 (2H, d, J 2.0, ArH);δ_(C)(100 MHz, CDCl₃) 12.0, 14.0, 22.5, 23.7, 31.3, 32.9, 122.2, 122.5,133.0, 140.4, 146.0; δ_(Si) (100 MHz, CDCl₃) 4.1.

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexyl-9H-9-dibenzosiloledibenzosilole

t-Butyllithium (1.19 cm³, 2.02 mmol, 1.7 M in Pentane) was added over 30minutes to a solution of 2,7-dibromo-9,9′-dihexyl-9H-9-dibenzosilole(0.25 g, 0.49 mmol) in dry THF (3 cm³) at −78° C. under nitrogenatmosphere. The mixture was stirred for a further 1 h at −78° C.2-Isopropoxy-4,4′, 5,5′-tetramethyl-1,3,2-dioxaboralane (0.25 cm³, 2.02mmol) was then added dropwise to the mixture and stirring continuedovernight at room temperature. The reaction was quenched with distilledwater, and the THF was removed by vacuum. The product was then extractedinto diethyl ether and the organic layer washed with brine, dried withanhydrous MgSO₄ and evaporated. Purification by column chromatography(hexane) using florisil yielded the title compound (0.22 g, 74%) as awhite solid (Found: C, 71.2; H, 9.5. C₃₆H₅₆Br₂O₄Si requires C, 71.8; H,9.4%); ν_(max)/cm⁻¹ (Neat liquid) 1093, 1143, 1345, 1597, 2922;δ_(C)(100 MHz, CDCl₃) 12.3, 14.1, 22.6, 23.8, 24.9, 31.3, 33.0, 83.7,120.5, 136.8, 137.5, 139.7, 151.0; δ_(Si) (100 MHz, CDCl₃) 3.2.

Monomers according to the second aspect of the invention mayalternatively be prepared according to the process set out below whereinthe intermediate compound2,2′-dibromo-4,4′-di(trimethylsilyl)-1,1′-biphenyl is firstly preparedaccording to the reaction scheme shown below:

The monomer according to the second aspect of the invention may bederived from this biphenyl intermediate by one of two routes:

Surprisingly, it is possible to perform selective transalkylation of thedimethylsilyl group obtained through Route B above without affecting thetrimethylsilyl end groups.

This allows for a wide number of substituents to be formed on thesilicon atom, for example hexyl and undecyl as illustrated in the abovescheme.

Polymer Examples Polymer Example 1

A homopolymer according to the first aspect of the invention wasprepared by Suzuki polymerization of Monomer 1 and Monomer 2 followed byend-capping with bromobenzene and phenylboronic acid according to thefollowing scheme to afford dibenzosilole polymer PS6:

To a dried Schlenk tube was added2,7-dibromo-9,9-dihexyl-9H-9-dibenzosilole (84 mg, 0.17 mmol, 1.0equiv.),2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9H,9-dihexyldibenzosilole(100 mg, 0.17 mmol, 1.0 equiv.), palladium(II) acetate (1.0 mg, 45 μmol,2.7%) and tricyclohexylphosphine (5 mg, 178 μmol, 10.7%) under nitrogenatmosphere. Dry toluene (2.5 cm³) was added and the mixture was stirredat 90° C. for 5 min. 20% w/w Tetraethylammonium hydroxide aqueoussolution (1.0 cm³) was then added. The mixture was stirred for a further2 h. To the mixture was then added phenylboronic acid (20.3 mg, 17 mmol,1.0 equiv.), and after stirring for 1 h, bromobenzene (26.1 mg, 17 mmol,1.0 equiv.) was added. After stirring for a further 1 h, the mixture wascooled to room temperature and poured into stirring methanol (30 cm³).The precipitate was dissolved in toluene (10 cm³) and reprecipitated instirring methanol (50 cm³). The precipitated product was filtered andthen dried in vacuo to yield the title compound (90 mg, 78%) as a palegrayish green solid. GPC assay in CHCl₃ vs. narrow polystyrene standardsrevealed M_(w)=8.7×10⁴, M_(n)=1.4×10⁴, M_(p)=1.0×10⁵, PDI=7.41;ν_(max)/cm⁻¹ (Neat solid) 731, 820, 1062, 1248, 1408, 1447, 2854, 2920,2955; δ_(H) (400 MHz, CDCl₃) 0.50-1.60 (m, CH₂+CH₃), 6.40-7.00 (brm,ArH), 7.40-8.10 (brm, ArH); δ_(C)(125 MHz, CDCl₃) 11.2, 14.1, 22.6,24.6, 31.4, 33.1, 121.2, 129.0, 131.8, 138.8, 139.9, 147.2; δ_(Si) (100MHz, CDCl₃) 3.07.

Polymer Example 2

A copolymer according to the invention was prepared by Suzukipolymerization as disclosed in WO 00/53656 with a diboronic acid ofdi(n-hexyl)fluorene followed by end-capping with bromobenzene and phenylboronic acid to afford Polymer PS6F6 as shown below:

Polymer PS6F6Poly(9,9-dihexyl-2,7-fluorenyl-alt-9,9-dihexyl-2,7-silafluorenyl)

To a dried Schlenk tube was added9,9-dihexyl-2,7-dibromo-9H-9-dibenzosilole (84 mg, 0.17 mmol, 1.0equiv.),2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene(97 mg, 0.17 mmol, 1.0 equiv.), palladium(II) acetate (1.0 mg, 45 μmol,2.7%) and tricyclohexylphosphine (5 mg, 178 μmol, 10.7%) under nitrogenatmosphere. Dry toluene (2.5 cm³) was added and the mixture was stirredat 90° C. for 5 min. 20% w/w Tetraethylammonium hydroxide aqueoussolution (1.0 cm³) was then added. After 1 hour, the mixture became veryviscous and additional dry toluene (1.0 cm³) was added. The mixture wasstirred for a further 1 h. To the mixture was then added phenylboronicacid (20.3 mg, 17 mmol, 1.0 equiv.), and after stirring for 1 h,bromobenzene (26.1 mg, 17 mmol, 1.0 equiv.) was added. After stirringfor a further 1 h, the mixture was cooled to room temperature and pouredinto stirring methanol (30 cm³). The precipitate was dissolved intoluene (10 cm³) and reprecipitated in stirring methanol (50 cm³). Theprecipitated product was filtered and then dried in vacuo to yield thetitle compound (100 mg, 93%) as a pale grayish green solid; GPC assay inCHCl₃ vs. narrow polystyrene standards revealed M_(w)=4.24×10⁵,M_(n)=1.09×10⁵, M_(p)=4.96×10⁵, PDI=4.55; ν_(max)/cm⁻¹ (Neat solid) 734,815, 1064, 1252, 1378, 1426, 1452, 2854, 2923, 2954; δ_(H)(500 MHz,CDCl₃) 0.74-0.89 (m, CH₂+CH₃), 1.00-1.20 (m, CH₂), 1.20-1.55 (m, CH₂),2.10 (brs, CCH₂), 7.50-8.00 (m, ArH); δ_(C) (125 MHz, CDCl₃) 12.4, 14.0,14.1, 22.56, 22.59, 23.8, 24.0, 29.7, 31.4, 31.5, 33.1, 40.4, 55.3,120.0, 121.2, 121.4, 125.3, 128.2, 129.2, 131.9, 138.9, 140.1 (2signals), 140.3, 147.1, 151.7; δ_(Si) (100 MHz, CDCl₃) 3.02.

Device Example

General Method:

Onto indium tin oxide supported on a glass substrate (available fromApplied Films, Colorado, USA) was deposited a layer of PEDT/PSS,available from H C Starck of Leverkusen, Germany as Baytron P®, by spincoating. A layer of electroluminescent polymer was deposited over thePEDT/PSS layer by spin-coating from xylene solution. Onto the layer ofelectroluminescent polymer was deposited by evaporation a cathodeconsisting of a first layer of calcium and a second, capping layer ofaluminium.

Devices according to the invention were made according to the abovemethod using Polymer PS6 and Polymer PS6F6.

For the purpose of comparison, a device was made according to the abovemethod comprising a layer of poly-9,9-di(n-hexyl)-2,7-fluorene(hereinafter referred to as Polymer PF6).

As can be seen from the table below, the PS6 homopolymer according tothe invention has a deeper LUMO level, i.e. it has a higher electronaffinity, than the corresponding polyfluorene homopolymer PF6. At thesame time, a wide HOMO-LUMO bandgap similar to that of PF6 is preserved.Furthermore, data for the PF6S6 polymer shows that a deep LUMO level ispreserved when the dibenzosilole units of the invention are conjugatedwith units having a shallower LUMO level.

Polymers E_(onset(ox)) (V)^(a) E_(g) ^(opt) (eV)^(b) HOMO (eV)^(c) LUMO(eV)^(d) PF6 1.41 2.93 −5.84 −2.91 PF6S6 1.48 2.92 −5.91 −2.99 PS6 1.522.93 −5.95 −3.02 ^(a)Oxidative onset potential. ^(b)Optical band gapenergy determined by the onset absorption (UV-Vis). ^(c)Determined fromE_(onset(ox)) (taking energy level of ferrocene to be −4.8 eV undervacuum). ^(d)Determined from adding E_(g) ^(opt) to the HOMO energylevel.

As can be seen from FIG. 2(a), photoluminescence of the prior art PF6polymer suffers from very significant color shift over time towards thered end of the visible spectrum. Incorporation of dibenzosilole repeatunits into the PF6 polymer, as shown in FIG. 2(b), results in a verysignificant reduction in this color shift, and color shift for the S6homopolymer, as shown in FIG. 2(c), is negligible.

As can be seen from FIG. 3, devices comprising PS6 or PS6F6 givesustained blue emission with emission maxima being identical to that ofphotoluminescence at 426 nm. The PF6 device on the other hand degradedvery rapidly under current and showed a green emission upon operation,displaying a broad peak at about 540 nm.

B) 3,6-Linked Dibenzosilole Monomers and Repeat Units

A monomer according to the ninth aspect of the invention was synthesisedaccording to the following reaction scheme. Again, as with the2,7-linked dibenzosiloles described above, transalkylation of thedimethylsilyl group (step e) does not affect the trimethylsilyl endgroups and so this route again provides a means for introducing a widerange of substituents on the silicon atom.

Scheme Synthesis of dibenzosilole monomers: a) n-BuLi, THF, −78° C. tort, 24 h, 88%; b) I₂, NaIO₄, conc. H₂SO₄, AcOH, Ac₂O, 24 h, 40%; c)n-BuLi, TMSCl, −78° C. to rt, 24 h, 84%; d) (i) t-BuLi, −78° C. to rt,24 h (ii) SiMe₂Cl₂, −78° C. to rt, 24 h, 81%; e) n-HexLi, −78° C., 15min, 95%; f) ICl, DCM, r.t., 1 h, 90%; g) t-BuLi,2-isopropoxy-4,4′,5,5′-tetramethyl-1,3,2-dioxaboralane, −78° C. to rt,24 h.

Scheme a) (i) P(OAc)₂, P(Cy)₃, Et₄NOH, Toluene, 90° C., 24 h (ii)phenylboronic acid, 2 h (iii) bromobenzene, 2 h, 51%.

For the purpose of comparison, the 2,7-linked polymerpoly(9,9-dihexyl-2,7-fluorenyl-alt-9,9-dihexyl-2,7-dibenzosilyl)(Polymer B) was prepared.

Poly(9,9-dihexyl-2,7-fluorenyl-alt-9,9-dihexyl-3,6-dibenzosilyl)(Polymer A) was prepared according to the following method:

Scheme a) (i) P(OAc)₂, P(Cy)₃, Et₄NOH, Toluene, 90° C., 24 h (ii)phenylboronic acid, 2 h (iii) bromobenzene, 2 h, 90%.

UV-Vis Absorption Spectra

The table below shows the absorption maxima and optical band gaps of thecopolymers. The solution UV-Vis absorption spectra of the copolymerswere measured in hexane.

λ_(max) (nm)^(a) E_(g) ^(opt) (eV, nm)^(b) Polymer solution filmsolution film PF6 394 390 2.98 (416) 2.93 (423) Polymer A 332 334 3.30(376) 3.23 (383) Polymer B 400 394 2.87 (432) 2.92 (425) ^(a)Opticalband gap, E_(g) ^(opt) (eV) = 1240/absorption edge (nm). ^(b)Band gapsof the polymers, measured from the UV absorption onsets.

As can be seen from the table, Polymer A comprising a 3,7-linkeddibenzosilole according to the eighth aspect of the invention issignificantly blue shifted and has a wider bandgap as compared to the2,7-linked poly-dibenzosilole Polymer B and the 2,7-linked polyfluorenePF6.

Cyclic Voltammetry

The films of the copolymers were prepared by spin-coating the samples(1.0 wt % solution in toluene) on the gold working electrode.Measurements were then taken in a solution of Bu₄N⁺ClO₄ ⁻ (0.10 M) inacetonitrile at a scan rate of 50 mV/s at room temperature, using aplatinum wire as the counter electrode and a Ag/AgCl electrode as thereference electrode. Both measurements were calibrated against ferrocenewhich has an ionization potential of 4.8 eV.

As can be seen from the table below, the 3,6-linked poly-dibenzosiloleof Polymer A according to the eighth aspect of the invention has a widerHOMO-LUMO bandgap as compared to 2,7-linked Polymer A.

Polymers E_(onset(ox)) (V)^(a) E_(g) ^(opt) (eV)^(b) HOMO (eV)^(c) LUMO(eV)^(d) Polymer A 1.57 3.23 −6.00 −2.77 Polymer B 1.48 2.92 −5.91 −2.99^(a)Oxidative onset potential. ^(b)Optical band gap energy determined bythe onset absorption (UV-Vis). ^(c)Determined from E_(onset(ox)) (takingenergy level of ferrocene to be −4.8 eV under vacuum). ^(d)Determinedfrom adding E_(g) ^(opt) to the HOMO energy level.

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that variousmodifications, alterations and/or combinations of features disclosedherein will be apparent to those skilled in the art without departingfrom the spirit and scope of the invention as set forth in the followingclaims.

1. A monomer comprising a repeat unit of formula (II):

wherein each R is the same or different and represents H or an electronwithdrawing group; each R1 is the same or different and represents asubstituent; and each X independently represents a polymerizable group.2. A monomer according to claim 1 wherein each X is the same ordifferent and is selected from the group consisting of boronic acidgroups, boronic ester groups, borane groups and halide functionalgroups.
 3. A monomer according to claim 1 wherein at least one R1 is asolubilizing group.
 4. A monomer according to claim 1 wherein each R1 isthe same or different and is independently selected from the groupconsisting of optionally substituted C1-20 alkyl, optionally substitutedC1-20 alkoxy, optionally substituted aryl and optionally substitutedheteroaryl.
 5. A monomer according to claim 1 wherein each R is H.
 6. Amethod of forming a polymer comprising the step of polymerizing amonomer according to claim
 1. 7. A method according to claim 6 whereinthe polymer is a copolymer formed by polymerizing the monomer with oneor more co-monomers.
 8. A method according to claim 7 wherein themonomer is polymerized with a co-monomer for forming an optionallysubstituted aryl or heteroaryl repeat unit.
 9. A method according toclaim 7 wherein the co-monomer is a co-monomer for forming an optionallysubstituted fluorene repeat unit.
 10. A method according to claim 7wherein the copolymer is an alternating copolymer.
 11. A methodaccording to claim 7 wherein the copolymer is a random copolymer.
 12. Amethod according to claim 7 wherein the copolymer is a block copolymer.13. A method according to claim 6 wherein each X is the same ordifferent and is a halide functional group, and the polymerization isperformed in the presence of a nickel complex catalyst.
 14. A methodaccording to claim 6 comprising the step of polymerizing: (a) a monomerof formula (II) wherein each X is a boron the same or different and is aboron derivative functional group selected from a boronic acid, aboronic ester and a borane, and an aromatic monomer having at least tworeactive halide functional groups; or (b) a monomer of formula (II)wherein each X is the same or different and is a reactive halidefunctional group, and an aromatic monomer having at least two boronderivative functional group selected from a boronic acid, a boronicester and a borane; or (c) a monomer of formula (II) wherein one X is areactive halide functional group and the other X is a boron derivativefunctional group selected from a boronic acid, a boronic ester and aborane, wherein the reaction mixture comprises a catalytic amount of apalladium catalyst suitable for catalyzing the polymerization of thearomatic monomers, and a base in an amount sufficient to convert theboron derivative functional groups into boronate anionic groups.